Although also suitable for other applications, the apparatus of the present invention is particularly useful in a system for treating contaminated soil containing volatile organic compounds. Such a system typically includes a heated rotary drum dryer or similar primary treatment unit that receives the contaminated soil and heats the same to a temperature sufficient to vaporize and drive off moisture and the volatile organic compounds therein. The exhaust gases and particulate products of combustion discharged from the primary treatment unit are next conducted to a cyclone or similar separator which separates fines from the exhaust gases. The latter then pass to a secondary treatment unit wherein the exhaust gases are heated to approximately 1100.degree.-1800.degree. F. This converts the contaminants mainly into carbon dioxide and water by high temperature oxidation. The gases discharged from the secondary treatment unit pass into a cooling chamber, wherein they are cooled, and thereafter into a baghouse wherein additional particulate matter is filtered from the gases before they are discharged into the atmosphere. Efficient utilization of a system of the foregoing type requires, among other things, that cooling of the extremely hot gases introduced into the cooling chamber be accomplished rapidly and reliably, notwithstanding the hostile high-temperature environment present within such chamber.